A two-way radio is a radio that can both transmit and receive (a transceiver) radio communications over a particular channel or frequency band. Two-way radios are often equipped with a push-to-talk (PTT) button that a user presses to initiate transmission of an analog radio communication to a recipient user. Such two-way radios are designed to operate according to a half-duplex system that provides for communication in both directions, but only in one direction at a time (i.e., not simultaneously). For instance, a two-way radio user initiates a radio communication to another user over a particular channel by pressing the PTT button on the transmitting user's two-way radio. The two-way radio of the recipient user must be tuned to the same frequency, and the recipient user must wait for the transmitting user to end his or her radio communication before the recipient user can reply. The transmitting user ends his or her radio communication by releasing the PTT button, which causes the channel to become available for the recipient user to transmit a reply radio communication. Once the transmitting user's radio communication is complete, the recipient user can then reply to the transmitting user over the same frequency by pressing the PTT button on his or her two-way radio to transmit a reply radio communication. The PTT button constitutes a change-of-state switch to switch the two-way radio from a voice transmission mode to a voice reception mode.
Two-way radios are widely used for personal, commercial and governmental purposes due to their ease of use and ability to transmit communications wirelessly. However, over-the-air radio communications are limited to particular geographic ranges.
Half-duplex radio communication functionality has been added to duplex cellular (mobile) phones in recent years to allow a cellular user to communicate with one or more other cellular users having a similarly equipped cellular radiotelephone via a half-duplex communication transmitted over a circuit switching network. Cellular half-duplex radio communication functions are often referred to as walkie-talkie or PTT PoC (push to talk over cellular) services. For instance, cellular phones have been equipped with a PTT button that a user can press to initiate a half-duplex radio communication with another user having a similarly equipped cellular phone, i.e., a radiotelephone. The initiating user first presses the PTT button on his or radiotelephone to initiate a radio communication to the intended recipient user over a cellular carrier network, and the base station of the cellular carrier network nearest to the recipient user, under the control of a central relay station of the cellular carrier, directs the half-duplex radio communication to the recipient user. The recipient user can then respond to the initiating user with a half-duplex radio communication by pressing the PTT button on the recipient user's radiotelephone. These half-duplex radio communications are transmitted as data over voice communications that are carried over a limited range of radio frequencies licensed to the cellular network carrier, such as by the Federal Communication Commission (FCC).
By pressing the PTT button on his or her radiotelephone, the initiating user requests the nearest base station of the cellular carrier network to assign the initiating user a particular radio frequency (channel) to transmit the half-duplex radio communication to the recipient user(s) on the voice network of the cellular carrier. However, due to the limited number of frequencies assigned to a particular cellular carrier network, there is often network congestion that prohibits the initiating user from acquiring a channel from the cellular carrier. Even if the initiating user is able to acquire a channel from the cellular carrier to transmit a half-duplex radio communication, the channel is released when the initiating user terminates the communication by releasing the PTT button on his or her cellular phone. The recipient user, to be able to reply to the initiating user's radio communication, must then acquire a channel from the cellular carrier. The congestion of a cellular carrier network is also affected by the number of mobile telephone “interconnect” calls made over the cellular carrier network. Unlike radio communications that operate according to a half-duplex system, mobile telephone calls, like land-line telephone calls, operate according to a full-duplex system. A full-duplex system allows communication in both directions simultaneously, which allows each caller to speak and be heard at the same time. Accordingly, each user on a mobile telephone call must therefore acquire an independent channel, which increases the congestion of the cellular carrier network. In addition, cellular PTT PoC services are presently supported only between users serviced by the same cellular carrier, meaning that users subscribing to different carriers are unable to transmit half-duplex radio communications to each other.
Many cellular carriers offer both voice and data services over the cellular carrier network to their subscribers. Voice services enable cellular subscribers to talk to other subscribers via full-duplex mobile communications and cellular half-duplex radio communications. Data services enable cellular subscribers to wirelessly connect to communication networks such as the Internet or private LANs in order to access websites and send and receive data such as email, text and chat messages, digital pictures and/or videos.
The technology of VoIP (Voice over Internet Protocol) has gained popularity in recent years. VoIP enables the transmission and reception of voice audio through the Internet or other packet switched networks. In VoIP, a software application operating on a computer or a hardware modem converts captured analog voice signals into one or more digital packets, and then transmits the digital packets to a recipient user over the Internet. A computer or modem of the recipient user converts the received digital packets back into analog audio data and outputs the analog audio data to an output device such as a speaker or telephone earpiece. VoIP thus provides a vehicle for moving data packets of voice audio from point-to-point via the Internet. VoIP provides full-duplex communication, similar to conventional land-line telephone services.
The technology of RoIP (Radio over Internet Protocol) has also gained popularity in recent years. RoIP is similar to VoIP in that RoIP also involves capturing analog voice signals, converting the analog signals into digital packets and then transmitting the digital packets through the Internet or other packet switched networks to one or more receiving devices. However, RoIP augments two-way radio communications rather than telephone calls. With RoIP, at least one node of a network is a radio connected via an Internet protocol (IP) to other nodes in the radio network. Accordingly, RoIP technology involves the conversion of half-duplex analog signals into digital packets and the subsequent transmission of the digital packets to a receiving device.
When converting a received analog signal into digital packets for VoIP or RoIP, the converting device identifies a source port and a destination port along with source and destination network addresses (e.g., IP addresses) in a header of the digital packet. Ports are 16-bit unsigned integers used to map data to a particular process running on a computer. A process may bind to a particular port to send and receive data, meaning that the process will listen for incoming packets whose destination port matches the port number to which it is bound, and send outgoing packets whose source port is set to the port number to which it is bound. Processes implementing common services will normally listen on specific port numbers which have been defined by convention for use with a given communication protocol. For instance, the Internet Assigned Numbers Authority (IANA) assigns port numbers to specific processes that communicate via protocols such as TCP (transmission control protocol) and UDP (user datagram protocol). For example, a server used for sending and receiving email may provide both an SMTP service for sending email and a POP3 service for receiving email. These services will be handled by different processes of the server, and the port number will be used to determine which data is associated with which process. By convention of the IANA, the SMTP server will listen on port 25, while the POP3 server will listen on port 110. Similarly, VoIP and RoIP applications are commonly assigned certain port numbers by convention.
However, cellular and satellite carriers regularly block ports dedicated to VoIP and RoIP applications. Accordingly, mobile terminal users are prohibited from operating VoIP and RoIP applications due to usage constraints placed on the users by the cellular and satellite carriers.